Role of the Size and Texture Properties of Copper-on-Alumina Pellets during the Simultaneous Removal of SO 2 and NO x from Flue Gas Gabriele Centi* and Siglinda Perathoner Department of Industrial Chemistry, University of Messina, Salita Sperone 31, 98166 Messina, Italy The role of the size and texture properties of industrial copper-on-alumina pellets for the simultaneous removal of SO 2 and NO x from flue gas on their behavior and kinetics of SO 2 capture is examined. The rate of reaction is influenced by both the intrinsic kinetics and rate of diffusion of SO 2 which in turn depends on the shape and texture properties of the pellets. A kinetic model, derived for small beads and based on the grain model modified to take into account the mechanism of SO 2 capture in the form of copper- and alumina-based surface sulfate species, was used for the design of pellets and the simulation of the behavior in fixed-bed flow reactor tests. The effectiveness of the sorbent-catalyst in the capture of SO 2 in flow reactor tests depends on the total volume and the mesoporous characteristics of the samples. Introduction The development of catalyst pellets for industrial applications often requires optimization of the texture properties of the solid catalyst in order to fit specific requirements for the pellet characteristics (size, shape, and mechanical strength) and rate of reaction. For their accurate design and optimization it is thus necessary to develop kinetic models which include both the intrinsic chemical kinetics and the intraparticle trans- port phenomena (Froment and Bischoff, 1990) and to develop advanced descriptions of the topology of the disordered porous medium such as those based on fractals (Coppens and Froment, 1995). Furthermore, in several cases the kinetic models should include a continuous alteration of the pore structure such as in the case of the presence of catalyst deactivation by carbon deposition (Froment, 1991). Noncatalytic gas-solid reaction is another example of a class of reactions of industrial importance in which (i) a chemical reaction leads to an alteration of the porous medium during the course of the reaction and (ii) the overall rate of reaction depends in several cases on both intrinsic kinetics and transport phenomena. The morphology of the solid, i.e., its connectedness (the way pores are connected to one another) and its geometry (the shapes and sizes of the pores), plays a fundamental role in transport and reactions in the system (Sahimi et al., 1990). Optimization of the behavior in gas-solid reactions thus requires an analysis of the role of the texture properties of the solid and the development of suitable kinetic models which can be used for the design of both reactors and solids with improved characteris- tics. A special case is when catalytic reactions occur simultaneously with the noncatalytic gas-solid reaction or a mixed catalytic and noncatalytic mechanism of reaction is present. An example of such a type of reaction is the simultaneous removal of SO 2 and NO x from flue gas using a copper-on-alumina sorbent- catalyst (Centi et al., 1990; Pollack et al., 1988; Yeh et al., 1985 and 1987; Yoo et al., 1994). A new dry technology based on a regenerable solid for the cleanup of emissions from combustion processes or the recovery of sulfuric acid from diluted ammonium sulfate solutions has been proposed based on the copper-on-alumina sorbent-catalyst (Centi et al., 1995; Paparatto et al., 1996). The objective of the present study was to show the role of the texture properties of the solid in the design of industrial copper-on-alumina pellets for this technology and to derive a kinetic model which can be used for both the simulation of the reactor performances and improvement of sorbent-catalyst behavior. The principle of the technology of SO 2 /NO x dry removal on a regenerable solid is the use of a sorbent- catalyst which is able to capture SO 2 (DeSOx) and at the same time catalyze the reduction of NO to N 2 (DeNO x ) at temperatures close to those of flue gas after the first economizer (around 350 °C). The SO 2 captured from the solid is then released in a separate stage to produce a concentrated stream of SO 2 which can be easily converted to sulfuric acid or sulfur with conven- tional processes. Copper-on-alumina sorbent-catalysts show distinct advantages for this process: 1. In the presence of the typical oxygen concentra- tions of flue gas and at temperatures above 300 °C they catalyze the oxidation of SO 2 to SO 3 which immediately reacts with the solid to form a surface sulfate species without deep bulk sulfation, if temperature is below 450 °C (Centi et al., 1992a; Waqif et al., 1991; Yoo et al., 1994). 2. They are active above 300 °C in the reduction of NO to N 2 in the presence of ammonia and O 2 and, in particular, in the presence of SO 2 , and the progressive formation of sulfate species on the surface has little influence on the activity (Centi et al., 1992c). 3. They can be easily regenerated using various reductants (CH 4 ,C 3 H 8 , CO, H 2 ) (Kartheuser et al., 1991). 4. They show stable reactivity toward SO 2 and NO x conversion in extended cyclic reaction-regeneration tests as well as maintain their texture and attrition resistance properties during lifetime experiments (Pa- paratto et al., 1996). 5. They can be produced at low cost in the industrial amounts required for commercial use. Economic estimations indicate that the preferable reactor configuration for the reaction step (capture of SO 2 and reduction of NO to N 2 ) is a radial-type mobile- bed reactor for its advantages in terms of low pressure * To whom correspondence should be addressed: e-mail, CENTI@SCIROCCO.UNIME.IT; fax, +39-90-391518; phone, +39-90-393134. 2945 Ind. Eng. Chem. Res. 1997, 36, 2945-2953 S0888-5885(96)00488-5 CCC: $14.00 © 1997 American Chemical Society